1. Field of the Invention
The invention concerns electret-enhanced filter media (more simply called "electret filters") made of fibers such as melt-blown microfibers or fibers obtained by fibrillating a film. The invention concerns fibrous electret filters for removing particulate matter from fluids and improved fibers for making those filters. The invention is especially concerned with respirators and the problem of improving the ability of filter media to accept filtration-enhancing electrostatic charges and to sustain that electret filtration enhancement in the presence of oily aerosols.
2. Description of the Related Art
For many years nonwoven fibrous filter webs have been made from polypropylene using melt-blowing apparatus of the type described in Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled "Manufacture of Super Fine Organic Fibers" by Van Wente et al. Such melt-blown microfiber webs continue to be in widespread use for filtering particulate contaminants, e.g., as face masks and as water filters. Melt-blown polypropylene microfiber webs are also used for other purposes, e.g., to remove oil from water as taught in U.S. Pat. No. 3,627,677 (Dyrud) and Re. 31,087 (Sohl). While the web of each example of those two patents is made of polypropylene microfibers, the latter patent says that adequate results are provided by microfibers of other polyolefin polymers, among which is "poly 4-methylpentene" that is more properly called poly(4-methyl-1-pentene) or poly-4-methylpentene-1. Poly(4-methyl-1-pentene) is available as "TPX" from Mitsui Petrochemical Industries, Ltd., and is sometimes here called "PMP".
U.S. Pat. No. 3,841,953 (Lohkamp et al.) says that nonwoven melt-blown microfiber webs can be made by melt-blowing a blend of two or more thermoplastic resins and that one of the advantages "is that an expensive polymer can be diluted to a certain degree with an inexpensive polymer without detracting too much from the gross properties of the expensive polymer." All the examples employ polypropylene as one of the resins, and in Example 3: "Pellet blends of poly-4-methylpentene-1 (TPX) were melt blown with Enjay CD 392 polypropylene resin" to provide "(v)ery good quality webs" or mats that "had improved temperature resistance as the amount of poly-4-methylpentene-1 increased."
U.S. Pat. No. 4,215,682 (Kubik et al.) teaches that the filtering efficiency of a melt-blown microfiber web can be improved by a factor of two or more when the "melt-blown fibers are bombarded as they issue from the orifices with electrically charged particles such as electrons or ions", thus making the fibrous web an electret. All of the examples employ melt-blown polypropylene microfibers which the Kubik patent says are especially useful, while saying that other polymers may also be used "such as polycarbonates and polyhalocarbons that may be melt-blown and have appropriate volume-resistivities under expected environmental conditions." Other techniques for providing electret enhancement are described in U.S. Pat. No 4,375,718 (Wadsworth); U.S. Pat. No. 4,588,537 (Klaase); and U.S. Pat. No. 4,592,815 (Nakao).
Fibrous filters for removing particulate contaminants from the air are also made from fibrillated polypropylene films. Electret filtration enhancement can be provided by electrostatically charging the film before it is fibrillated as taught in U.S. Pats. No. Re. 30,782 (Van Turnhout) and Re. 31,285 (Van Turnhout et al.).
U.S. Pat. No. 4,288,584 (Mishra) says that "common polymers such as polyesters, polycarbonates, etc. can be formulated initially as highly charged electrets but these charges are short-lived especially under humid conditions... It has also been proposed to make electrets from simple polyolefins such as polyethylene and polypropylene... Unfortunately such electrets are not very stable" (col. 1, lines 22-31). Mishra discovered that "a stable electret can be obtained from a resinous polymer of an alpha-olefin having a branched side chain which polymer has a degree of crystallinity of at least 20% and is composed of repeating units having the structure ##STR1## where R is a branched alkyl radical having from 3 to 10 carbon atoms." The electret structures mentioned in the Mishra patent are films or sheets which "find applications as the electrostatic element in electro-acoustic devices such as microphones, headphones and speakers and in dust particle control, high voltage electrostatic generators, electrostatic recorders and other applications" (col. 3, lines 37-43).
Fibrous polypropylene electret filters that are currently on the market, some made from melt-blown polypropylene microfibers and others from fibrillated polypropylene film, show stable electret filtration enhancement under the test conditions of the Mishra patent. From Mishra's test data, PMP would appear to offer no advantage in fibrous electret filters, and no fibrous electret filter made of PMP is known to be on the market. Of the polyolefins coming within the above-reproduced formula of the Mishra patent, only PMP is commercially available, and raw materials for making any of the other polyolefins currently cost far more than do those from which PMP is made.
Unfortunately, fibrous electret filters made of polypropylene, whether melt-blown microfibers or fibrillated film, tend to lose their electret enhanced filtration efficiency faster than desired for some purposes when exposed to aerosol oils. There is a growing awareness of the need to improve the long-term efficiency of air filters in the presence of aerosol oils, especially in respirators. 30 C.F.R. Part 11, subpart K, describes current protocols for testing particulate air filters with an oily aerosol challenge of dioctyl phthalate (DOP) particles.